1 前言

        使用Mesh绘制三角形 中介绍了绘制三角形的方法，使用Mesh绘制矩形 中介绍了绘制矩形的方法，本文将介绍绘制圆形的方法。

        libGDX 以点、线段、三角形为图元，没有提供绘制圆形的接口。要绘制圆形边框，必须通过割圆法逼近圆形；要绘制圆形的内部，必须通过三角形逼近圆形，如下图，是通过 GL_TRIANGLE_FAN 模式逼近圆形。

2 绘制圆形

        DesktopLauncher.java

package com.zhyan8.game;import com.badlogic.gdx.backends.lwjgl3.Lwjgl3Application;
import com.badlogic.gdx.backends.lwjgl3.Lwjgl3ApplicationConfiguration;public class DesktopLauncher {public static void main (String[] arg) {Lwjgl3ApplicationConfiguration config = new Lwjgl3ApplicationConfiguration();config.setForegroundFPS(60);config.setTitle("Circle");new Lwjgl3Application(new Circle(), config);}
}

        Circle.java

package com.zhyan8.game;import com.badlogic.gdx.ApplicationAdapter;
import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.graphics.GL30;
import com.badlogic.gdx.graphics.Mesh;
import com.badlogic.gdx.graphics.VertexAttribute;
import com.badlogic.gdx.graphics.VertexAttributes.Usage;
import com.badlogic.gdx.graphics.glutils.ShaderProgram;public class Circle extends ApplicationAdapter {private ShaderProgram mShaderProgram;private Mesh mMesh;private float mRatio;@Overridepublic void create() {initShader();initMesh();}@Overridepublic void render() {Gdx.gl.glClearColor(0.455f, 0.725f, 1.0f, 1.0f);Gdx.gl.glClear(GL30.GL_COLOR_BUFFER_BIT);mShaderProgram.bind();mShaderProgram.setUniformf("u_wh_ratio", mRatio);mMesh.render(mShaderProgram, GL30.GL_TRIANGLE_FAN); // 绘制圆形内部// mMesh.render(mShaderProgram, GL30.GL_LINE_LOOP); // 绘制圆形框}@Overridepublic void dispose() {mShaderProgram.dispose();mMesh.dispose();}private void initShader() { // 初始化着色器程序String vertex = Gdx.files.internal("shaders/circle_vertex.glsl").readString();String fragment = Gdx.files.internal("shaders/circle_fragment.glsl").readString();mShaderProgram = new ShaderProgram(vertex, fragment);mRatio = 1.0f * Gdx.graphics.getWidth() / Gdx.graphics.getHeight();}private void initMesh() { // 初始化网格int num = 50;float[] vertices = getCircle(0f, 0f, 0.5f, num);short[] indices = getIndices(num);VertexAttribute vertexPosition = new VertexAttribute(Usage.Position, 3, "a_position");mMesh = new Mesh(true, vertices.length / 3, indices.length, vertexPosition);mMesh.setVertices(vertices);mMesh.setIndices(indices);}private float[] getCircle(float centerX, float centerY, float radius, int num) {float unit = (float)(2 * Math.PI / num);float[] coords = new float[num * 3];for (int i = 0; i < num; i++) {coords[i * 3] = (float)(centerX + radius * Math.cos(unit * i));coords[i * 3 + 1] = (float)(centerY + radius * Math.sin(unit * i));coords[i * 3 + 2] = 0;}return coords;}private short[] getIndices(int num) {short[] indices = new short[num];for (short i = 0; i < num; i++) {indices[i] = i;}return indices;}
}

        circle_vertex.glsl

#version 300 esin vec3 a_position;uniform float u_wh_ratio; // 屏幕宽高比void main() {gl_Position = vec4(a_position, 1.0);if (u_wh_ratio > 1.0) {gl_Position.x /= u_wh_ratio;} else {gl_Position.y *= u_wh_ratio;}
}

        circle_fragment.glsl

#version 300 es
precision mediump float; // 声明float型变量的精度为mediumpout vec4 fragColor;void main() {fragColor = vec4(1, 0, 0, 0);
}